1. Field of the Invention
The present invention relates to an in-line electron gun of a color cathode ray tube, more particularly relates to an improvement in the electrode structure in a main lens electric field generating portion of an in-line electron gun of a color cathode ray tube.
2. Description of the Related Art
In recent years, in order to improve the resolution at a peripheral portion of a screen of a cathode ray tube, wide use has been made of cathode ray tubes using the common electric field system for the main lens of the in-line electron gun. In such an in-line electron gun, enlargement of the diameter of the main lens is achieved by forming a focusing electrode constituting the main lens and an accelerating electrode adjoining this by a cylindrical metal member having an elliptical cross-section having an opening through which three electron beams can pass.
In the in-line electron gun described above, since the shape of the opening of the focusing electrode and the accelerating electrode adjoining this is elliptical, the electric field in the main lens is asymmetrically distorted, aberration such as spherical aberration, astigmatism, or frame aberration occurs in the main lens and exerts an adverse influence upon the focusing characteristics etc. of the electron gun.
As a method for reducing the effect of the degree of the aberration, for example, a method of providing a field-correcting electrode plate comprised of a metal plate for correcting the electric field of the main lens along an opening direction of an internal portion of each of the focusing electrode and the accelerating electrode adjoining this has been known. This field-correcting electrode plate has three through apertures through which the electric beam may pass arranged in-line in the long axis direction of the elliptically shaped metal plate. Correction and adjustment of the aberration in the main lens is possible by adjustment of the shape of the through apertures, for example, not making the shape of the through apertures circular, but a special shape such as an ellipse, for example, suitably changing the diameter in the lateral direction and the diameter in the vertical direction.
However, in order to improve the performances of the in-line electron gun, it is necessary to raise the assembly precision at the time of assembly of the main lens. When positioning the field-correcting electrode plate with respect to the path of the electron beam, when the through apertures on both sides are circular in shape, high precision positioning is possible by inserting circular inner core guides into the through apertures, but when the above field-correcting electrode plate is used, since the through apertures do not have a circular shape, inner core guides having circular cross-sections for insertion into the through apertures cannot be used and therefore it was difficult to control the precision of positioning, particularly the precision of positioning in the rotation direction of the metal plate.
A method for solving the above problem has been proposed in for example Japanese Examined Patent Publication (Kokoku) No. 6-75378. In this method, by forming the center through aperture among the three through apertures of the field-correcting electrode plate as an elliptical aperture, forming the through apertures on the two sides as circular apertures, setting an aspect ratio of the elliptically shaped center through aperture of the field-correcting electrode plate within a predetermined range, and inserting circular inner core guides into the through apertures on the two sides at the time of assembly of the electron gun, high precision positioning can be carried out and, at the same time, distortion of the electric field of the main lens can be corrected. According to Japanese Examined Patent Publication (Kokoku) No. 6-75378, since the through apertures on the two sides of the field-correcting electrode plate are formed as circular apertures, it is easy to raise the assembly precision of the in-line electron gun, but since adjustment of the astigmatism is not possible, a corrective metal plate etc. for adjusting the astigmatism newly becomes necessary, therefore there was the continued disadvantage that the structure of the main lens became complex and also the assembly process became complex.